1. Field of the Invention
The present invention relates to dialysis machines and in particular, but not exclusively, to a method of preparing a dialysate solution for use in dialysis.
2. State of the Art
Dialysis is a treatment which replaces the renal function of removing excess fluid and waste products, such as potassium and urea, from blood. The treatment is either employed when renal function has deteriorated to an extent that uremic syndrome becomes a threat to the body's physiology (acute renal failure) or, when a longstanding renal condition impairs the performance of the kidneys (chronic renal failure).
There are two major types of dialysis, namely hemodialysis and peritoneal dialysis.
In peritoneal dialysis treatment, a dialysate solution is run through a tube into the peritoneal cavity. The fluid is left in the cavity for a period of time in order to absorb the waste products, and is subsequently removed through the tube for disposal.
It is common for patients in the early stages of treatment for a longstanding renal condition to be treated by peritoneal dialysis before progressing to hemodialysis at a later stage.
In hemodialysis, the patient's blood is removed from the body by an arterial line, is treated by the dialysis machine, and is then returned to the body by a venous line. The machine passes the blood through a dialyser containing tubes formed from a semi permeable membrane. On the exterior of the semi permeable membrane is a dialysate solution. The semi permeable membrane filters the waste products and excess fluid from the blood into the dialysate solution. The membrane allows the waste and a controlled volume of fluid to permeate into the dialysate whilst preventing the loss of larger more desirable molecules, like blood cells and certain proteins and polypeptides.
The action of dialysis across the membrane is achieved primarily by a combination of diffusion (the migration of molecules by random motion from a region of higher concentration to a region of lower concentration), and convection (solute movement that results from bulk movement of solvent, usually in response to differences in hydrostatic pressure).
Fluid removal (otherwise known as ultrafiltration) is achieved by altering the hydrostatic pressure of the dialysate side of the membrane, causing free water to move across the membrane along the pressure gradient.
The correction of uremic acidosis of the blood is achieved by use of a bicarbonate buffer. The bicarbonate buffer also allows the correction of the blood bicarbonate level.
The dialysis solution consists of a sterilized solution of mineral ions. These ions are contained within an acid buffer which is mixed with water and bicarbonate base prior to delivery to the dialyser. The water used is cleaned to a sufficient degree that it is suitable for use as a base for trans-membrane ion transfer with the blood (hereinafter sterile water), this may for example be achieved by known methods including reverse osmosis, heat treatment, filtration or a combination of such known methods.
Dialysate composition is critical to successful dialysis treatment since the level of dialytic exchange across the membrane, and thus the possibility to restore adequate body electrolytic concentrations and acid-base equilibrium, depends on the composition.
The correct composition is accomplished primarily by formulating a dialysate whose constituent concentrations are set to approximate normal values in the body.
However, achieving the correct composition of dialysate requires the accurate control of low volumes of liquid and at present this is achieved by the provision of complex fluid paths, including multiple pumping and valving components on the dialysis machine.
This presents the disadvantage of a complex and costly dialysis machine which is at increased risk of failure by virtue of its complexity. Increased maintenance is also a problem since it is essential to minimise machine downtime in order to most efficiently treat the patient.
A further problem with known hemodialysis machines is that the blood and dialysate solution lines require careful mounting onto the dialysis machine before the treatment can commence. This presents a risk that the lines are not correctly installed, a risk which is particularly relevant to those patients who dialyse at home.
This method of dialysis also presents an increased risk of cross-infection between patients since the disposable blood and dialysate lines come into contact with the dialysis machine.
The problems associated with conventional dialysis equipment are mitigated to some degree by the system disclosed in WO 2006/120415 which discloses a cartridge based system for conducting hemodialysis, however the method and system for mixing the dialysate proposed in this application is complex and costly involving a large cartridge with multiple reservoirs, each having level control and therefore requiring a complex pumping and control system. Both this complexity and this space requirement are undesirable in portable dialysis machines, for example those suitable for home dialysis.